Nowadays and in the coming several decades, coal will be the main source of energy in China. However, coal has properties different from those of gaseous fuels; coal contains a great amount of harmful substances such as ash and the like, while the gaseous fuels are clean energy sources. Just because the differences between coal and gaseous fuels in term of the quality as well as the energy conversion and utilization mode, the power generation efficiency of a coal-fired power plant is 10%-15% lower than that of a natural gas combined cycle power plant. Currently, in order to achieve an efficient and clean utilization of coal, coal must be gasified in advance so as to provide necessary conditions for the efficient utilization of coal, whether in a production process in the chemical industry of coal or in the integrated gasification combined cycle (IGCC) power generation system.
The gasification process of coal is a technical process, in which coal or coal coke is used as a raw material; oxygen (air, oxygen-rich gas or industrial pure oxygen), carbon dioxide, water vapor or the like is used as a gasification agent (or referred to as a gasification medium); the combustible portion of the coal or coke oven gas is transformed into a combustible gas by chemical reactions in a high temperature condition. The composition of the syngas or coal gas obtained by coal gasification depends on the type of the fuels and the gasification agent, as well as the conditions for performing the gasification process. There are variety of coal gasification processes. Four kinds of gasifiers used widely are the Lurgi furnace, Winkler furnace, Texaco furnace and Shell furnace. The cold coal gas efficiency thereof is about 65%-80%.
An air separation unit is needed due to the need for pure oxygen to react with coal in the coal gasification process. The resultant syngas only can be used after cleaning and removing dusts, and a waste heat boiler is also necessary to recover the waste heat, which renders the coal gasification efficiency low and the cost of investment very high.
Meanwhile, different coal gasification processes have different requirements depending on the types of coal. In China, the mineable coal is about one trillion tons, being divided into lignite, bituminous coal and anthracite. Lurgi gasifier can utilize lignite and a part of bituminous coal (long-flame coal, non-caking coal, weakly caking coal and gas coal); the available amount of resources is 593.45 billion tons and accounts for 58% of the total amount of resources. Texaco gasifier can utilize most of bituminous coal; the available amount of resources is 672.88 billion tons and accounts for 66% of the total amount of resources. Shell gasifier can utilize lignite and almost all of bituminous coal; the available amount of resources is 924.2 billion tons and accounts for 91% of the total amount of resources. Winkler gasifier can utilize lignite and a small part of bituminous coal (long-flame coal and non-caking coal); the available amount of resources is 447.96 billion tons and accounts for 44% of the total amount of resources.
In China, the highly efficient and clean utilization of coal is a difficult problem to be solved urgently. Currently, technique used to achieve this objective in the electric power industry is to use the integrated gasification combined cycle (IGCC) power generation system. The IGCC system first gasifies the coal to create necessary conditions for further utilization of coal, and then generates power by using the syngas obtained from the coal gasification.
However, unlike the coal-fired power plant, the IGCC system has strict requirements on the types of coal, which depends on the mode of coal gasification adopted by the IGCC system. The IGCC system has problems such as low gasification efficiency and high equipment costs. The low gasification efficiency is mainly due to the need for adding pure oxygen during the coal gasification. The energy consumption of the air separation device for producing oxygen is high, meanwhile the equipment investments are also increased. The high equipment costs are mainly due to the current relatively high equipment cost of the gasification unit. Because more ash and sulfur are contained in the gasified syngas, dust removal and desulfuration units are needed, which makes the equipment investments to be further increased. Currently, the cost of IGCC is about 1500$/kW, in which the air separation device accounts for about 10-20%, the gasification device accounts for about 30%, the waste heat boiler accounts for about 10-15%, the combined cycle power generation unit accounts for about 30%, and the syngas cleaning unit accounts for about 10%.
Additionally, the main types of coal for the traditional coking technique are coking coal and fat coal in the bituminous coal, which account for only about 9% of the total resources.
To sum up, in the gasification process of coal and in the promotion of application of IGCC power generation system, there are still obstacles such as low gasification efficiency, high costs of equipment investment, and limitation in types of coal. Therefore, either for the chemical industry or for the power generation industry, how to improve the efficiencies in coal gasification and power generation, how to reduce the costs of coal gasification and power generation, and how to improve the adaptability of the power generation system of coal gasification to the types of coal, are all huge challenges to be solved urgently.